1. Field of the Invention
The present invention relates to an image reading apparatus with shading correction, which is utilized in a digital copying machine obtained by connecting, e.g., a laser printer and an image scanner.
2. Description of the Related Art
In recent years, a digital copying machine (image reading apparatus) obtained by connecting a laser printer (image forming unit) for forming an image by an electrophotography system, and an image scanner (reading unit) for reading image information of an original or manuscript as image data upon scanning of an optical carriage (scanning means) has been put into practical applications.
As an original illumination device disposed on an optical carriage of an image scanner in a copying machine of this type, a fluorescent lamp is widely used since it has the advantages of low power consumption, a relatively uniform spectrum distribution over a wide wavelength range, a small temperature rise, and the like.
However, a fluorescent lamp has the following adverse characteristics. That is, the light amount is changed due to a variation in temperature, or the light amount distribution in the longitudinal direction of the lamp is changed. For this reason, when a fluorescent lamp is used as an original illumination device, the temperature of the fluorescent lamp is controlled to be constant by arranging a heater around the fluorescent lamp except for a portion opposing an original as a light exit, or main scan shading correction data is rewritten prior to read scanning of an original. Thus, the change in light amount or in light amount distribution is canceled, and the accuracy (reliability) of image data with respect to an original image during one scanning period is assured.
A conventional digital copying machine which employs a fluorescent lamp as an original illumination device must employ a heater for heating the fluorescent lamp, and a temperature control circuit, resulting in an increase in cost. Before the temperature of the fluorescent lamp is stabilized, read scanning of an original cannot be performed, and main scan shading correction data must be rewritten for every read scanning.
In particular, a laser printer (page printer) itself can perform image formation at high speed (high-speed printing operation) if image data are sequentially input. However, the image scanner suffers from an operation which does not contribute to the above-mentioned image formation (rewriting of main scan shading correction data). This is one of the factors which prevent a further increase in image formation speed of a digital copying machine.